Electroluminescent lighting has been known in the art for many years as a source of light weight and relatively low power illumination. Because of these attributes, electroluminescent lamps are in common use today providing light for displays in, for example, automobiles, airplanes, watches, and laptop computers. One such use of electroluminescence is providing the back light necessary to view Liquid Crystal Displays (LCD).
Electroluminescent lamps may typically be characterized as "lossy" parallel plate capacitors of a layered construction. Electroluminescent lamps of the current art generally comprise a dielectric layer and a luminescent layer separating two electrodes, at least one of which is translucent to allow light emitted from the luminescent layer to pass through. The dielectric layer enables the lamp's capacitive properties. The luminescent layer is energized by a suitable power-supply, typically about 115 volts AC oscillating at about 400 Hz, which may advantageously be provided by an inverter powered by a dry cell battery. Electroluminescent lamps are known, however, to operate in voltage ranges of 60 V-500 V AC, and in oscillation ranges of 60 Hz-2.5 KHz.
It is standard in the art for the translucent electrode to consist of a polyester film "sputtered" with indium-tin-oxide (ITO). Typically, the use of the polyester film sputtered with ITO provides a serviceable translucent material with suitable conductive properties for use as an electrode.
A disadvantage of the use of this polyester film method is that the final shape and size of the electroluminescent lamp is dictated greatly by the size and shape of manufacturable polyester films sputtered with ITO. Further, a design factor in the use of ITO sputtered films is the need to balance the desired size of electroluminescent area with the electrical resistance (and hence light/power loss) caused by the ITO film required to service that area. Generally, a large electroluminescent layer will require a low resistance ITO film to maintain manageable power consumption. Thus, the ITO sputtered films must be manufactured to meet the requirements of the particular lamps they will be used in. This greatly complicates the lamp production process, adding lead times for customized ITO sputtered films and placing general on the size and shape of the lamps that may be produced. Moreover, the use of ITO sputtered films tends to increase manufacturing costs for electroluminescent lamps of nonstandard shape.
The other layers found in electroluminescent lamps in the art are suspended in a variety of diverse carrier compounds (often also referred to as "vehicles") that typically differ chemically from one another. As will be described, the superimposition of these carrier compounds upon one another and on to the sputtered ITO polyester film creates special problems in the manufacture and performance of the lamp.
The electroluminescent layer typically comprises an electroluminescent grade phosphor suspended in a cellulose-based resin in liquid form. In many manufacturing processes, this suspension is applied over the sputtered ITO layer on the polyester of the translucent electrode. Individual grains of the electroluminescent grade phosphor are typically of relatively large dimensions so as to provide phosphor particles of sufficient size to luminesce strongly. This particle size, however, tends to cause the suspension to be non-uniform. Additionally, the relatively large particulate size of the phosphor can cause the light emitted from the electroluminescent to appear grainy.
The dielectric layer typically comprises a titanium dioxide and barium-titanate mixture suspended in a cellulose-based resin, also in liquid form. Continuing the exemplary manufacturing process described above, this suspension is typically applied over the electroluminescent layer. It should be noted that for better luminescence, the electroluminescent layer generally separates the translucent electrode and the dielectric layer, although those in the art will understand that this is not a requirement for a functional electroluminescent lamp. It is possible that unusual design criteria may require the dielectric layer to separate the electroluminescent layer and the translucent electrode. It should also be noted that, occasionally, both the phosphor and dielectric layers of the lamps in the art utilize a polyester-based resin for the carrier compound, rather than the more typical cellulose-based resin discussed above.
The second electrode is normally opaque and comprises a conductor, such as silver and/or graphite, typically suspended in an acrylic or polyester carrier.
A disadvantage of the use of these liquid-based carrier compounds standard in the art is that the relative weight of the various suspended elements causes rapid separation of the suspension. This requires the frequent agitation of the liquid solution to maintain the suspension. This agitation requirement adds a manufacturing step and a variable to suspension quality. Furthermore, liquid carrier compounds standard in the art tend to be highly volatile and typically give off noxious or hazardous fumes. As a result, the current manufacturing process must expect evaporative losses in an environment requiring heightened attention to worker safety.
A further disadvantage in combining different carrier compounds, as is common in the art, is that the bonds and transitions between the multiple layers are inherently radical. These radical transitions between layers tend strongly to delaminate upon flexing of the assembly or upon exposure to extreme temperature variations.
A still further disadvantage in combining different carrier compounds is that different handling and application requirements are created for each layer. It will be appreciated that each layer of the electroluminescent lamp must be formed using different techniques including compound preparation, application, and curing techniques. This diversity in manufacturing techniques complicates the manufacturing process and thus affects manufacturing cost and product performance.
A need in the art therefore exists for an electroluminescent system in which the layers are suspended in a unitary common carrier. A structure would thereby be created in which, once cured, layers will become strata in a monolithic mass. Manufacturing will thus tend to be simplified and product performance will tend to improve.